What If … the “Hockey Stick” Were Wrong?

The “hockey stick” reconstruction of temperatures of the past millennium has attracted much attention – partly as it was high-lighted in the 2001 IPCC report as one of the important new results since the previous IPCC report of 1995, and partly as it has become the focus of a number of challenges. Discussion about the “hockey stick” is conducted with considerable fervor in the public media, where this curve is often presented as if it were a proof, or even the most important proof, of anthropogenic influence on climate.

As someone who has not worked on the past millennium, I do not want to discuss the merits of the often rather technical challenges (which have been dealt with elsewhere on this site). Rather, I want to discuss the “what if…” question: what if really some serious flaw was discovered in the “hockey stick” curve? What would that mean?

So let’s assume for argument’s sake that Mann, Bradley and Hughes made some terrible mistake in their statistical analysis, so we need to discard their results altogether. This wouldn’t change our picture of the last millennium (or anything else) very much: independent groups, with different analysis methods, have arrived at similar results for the last millennium. The details differ (mostly within the uncertainty bounds given by Mann et al, so the difference is not significant), but all published reconstructions share the same basic features: they show relatively warm medieval times, a cooling by a few tenths of a degree Celsius after that, and a rapid warming since the 19th Century. Even without Mann et al, we’d still be stuck with a “hockey stick” type of curve – quite boring.

So let’s try some more exciting “what ifs”. In mid-20th Century, medieval temperatures are exceeded in all the reconstructions, hence recent (last 10-15 years, say) temperatures appear to be unprecedented for at least a millennium (that even holds for the alternative histories presented by the “hockey stick” critics). Now what if that were wrong – if all proxy reconstructions as well as model simulations of the past millennium were fundamentally in error?

Let us assume that medieval temperatures after all had been warmer than the present. Even that would tell us nothing about anthropogenic climate change. The famous conclusion of the IPCC, “The balance of evidence suggests that there is a discernible human influence on global climate”, does not depend on any reconstruction for the past millennium. It depends on a detailed analysis of 20th Century data. In fact, this conclusion is from the 1995 IPCC report, and thus predates the existence of quantitative proxy reconstructions like the “hockey stick”.

Climate changes can have several different reasons, and the cause of any particular climate change needs to be investigated on a case by case basis. It cannot be found by looking at one temperature curve. Had medieval climate been warmer than the present, this would probably have been due to some natural cause – perhaps a peak in solar output. That would only tell us that in principle, natural causes can cause warming larger than what we’ve seen in the past decades. But we know that already – one need only go back far enough in time (e.g., fifty million years) to find examples of unquestionably warmer climates than today. However, it would be naive to conclude that the observed strong 20th Century warming therefore also must have a natural cause.

Investigating the cause of 20th Century warming is done in so-called detection and attribution studies, which analyze the various forcings (e.g., solar variations, greenhouse gases or volcanic activity) and the observed time and space patterns of climate change in detail. These studies, with a range of different techniques, have invariably concluded that the dominant cause of 20th Century warming is man-made greenhouse gases.

In the spirit of this article, let’s assume these studies were also wrong, in addition to all of the above. Let’s assume these studies somehow greatly underestimated natural variability in the climate system, so that the “signal” of anthropogenic climate change has not yet emerged from the “noise” of natural variations (i.e., the above-cited “discernible human influence” had not been detected after all). Surely, then we wouldn’t need to worry about global warming, and the world could hold off with the Kyoto protocol?

Unfortunately, that also doesn’t follow. The only thing that would follow in that case is that our data are not yet good enough to prove that anthropogenic climate change is already happening. That would not be so surprising – the expected amount of anthropogenic global warming to date (based on the radiative effects of the greenhouse gases and aerosols emitted by humans thus far) is only ~0.5 ºC. It is a small signal that is not easy to detect amongst the natural variability; most of the anthropogenic warming is still to come (the point of conducting science is to give an early warning, rather than just wait until the facts are obvious to everyone).

The main reason for concern about anthropogenic climate change is not that we can already see it (although we can). The main reason is twofold.
(1) Carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity. This is a measured fact not even disputed by staunch “climate skeptics”.
(2) Any increase in carbon dioxide and other greenhouse gases will change the radiation balance of the Earth and increase surface temperatures. This is basic and undisputed physics that has been known for over a hundred years.

But how strong is this warming effect? That is the only fundamental doubt about anthropogenic climate change that can still be legitimately debated. We climatologists describe this in terms of the climate sensitivity, the warming that results in equilibrium from a doubling of CO2. The IPCC gives the uncertainty range as 1.5-4.5 ºC. Only if this is wrong, and the true value is lower, can we escape the fact that unabated emissions of greenhouse gases will lead to the warming projected by the IPCC.

Chances for that are not good. A new large uncertainty analysis that appeared this week in Nature shows that it is very difficult to get a climate sensitivity below 2 ºC in a climate model, no matter how one changes the parameters. And climate history, with its Ice Ages and other large changes, also speaks strongly against low climate sensitivity.

Discussion about the temperature evolution of the past millennium will no doubt continue in the coming years. The most fundamental problem – the sparseness of data – will not be fixed quickly, but eventually better reconstructions with smaller uncertainties will become available. However, this discussion needs to be conducted in a sober and unexcited manner; it does not help to overburden the “hockey stick” with symbolic meaning. In some media reports, the “hockey stick” has even been hyped as “a pillar of the Kyoto protocol” (which was agreed in 1997 and thus predates it) or as “proof that humans are warming the Earth”. This is a serious misunderstanding of the scientific meaning of these data.

The discussions about the past millennium are not discussions about whether humans are changing climate; neither do they affect our projections for the future. In fact, if humanity takes no action and this century will bring a temperature rise of 2 ºC, 3 ºC or even more, the current discussions over whether the 14th Century was a few tenths of a degree warmer or the 17th a few tenths cooler than previously thought will look rather academic.

35 Responses to “What If … the “Hockey Stick” Were Wrong?”

So let’s try some more exciting “what ifs”. In mid-20th Century, medieval temperatures are exceeded in all the reconstructions, hence recent (last 10-15 years, say) temperatures appear to be unprecedented for at least a millennium (that even holds for the alternative histories presented by the ‘hockey stick’ critics).

Most proxy-only reconstructions show the mid-20th century (not late) to be the warmest period – which we know, according to surface temperature observations, is incorrect. Towards the end of the 20th century the proxy reconstructions dip and certainly don’t reproduce anywhere near the warming indicated by the surface temperature record.

the expected amount of anthropogenic global warming to date (based on the radiative effects of the greenhouse gases and aerosols emitted by humans thus far) is only ~0.5 ºC.

0.5 seems a bit high.Your models appear to support the assertion that the warming in the early part of the 20th century was entirely natural. See diagram A here.

But how strong is this warming effect? That is the only fundamental doubt about anthropogenic climate change that can still be legitimately debated. We climatologists describe this in terms of the climate sensitivity, the warming that results in equilibrium from a doubling of CO2. The IPCC gives the uncertainty range as 1.5-4.5 ºC

I think the IPCC now gives the range as 1.4- 5.8, but let’s not worry unduly about that

This http://www.giss.nasa.gov/data/simodel/F_line.gif suggests an increased climate forcing (net) of about 2 W/m2 since the start of the 20th century which has resulted in about 0.35 deg C rise (not 0.5) – but let’s call it 0.4 deg C – which implies a climate sensitivity of about 0.2 deg C per W/m2. A doubling of CO2 is supposed to produce additional climate forcing of around 4 W/m2. So a doubling of CO2 should result in a rise of about 0.8 deg C – pretty close to Michael Crichton’s estimate.

If anyone’s running a book on Crichton v IPCC – I’ll have a couple of hundred on Crichton

[Response: I would recommend you not be so free with your lira. You confuse the IPCC climate sensitivity range (1.5-4.5 C) with the estimates of temperatures in 2100 (1.4 to 5.8 C). The range of the latter is larger than the former because of the uncertainty in the projected levels of greenhouse gases etc. Your attempt to estimate equilibrium climate sensitivity from the 20th C won’t work because a) the forcings are not that well known (so the error in your estimate is large), b) the climate is not in equilibrium – you need to account for the uptake of heat in the ocean at least. I have a post on this exact issue – gavin]

The introductory paragraphs of the new Stainforth et al. article mention that the climateprediction.net modeling includes “a mixed-layer ocean”. In the methods section, it’s described like this: “The ocean consists of a single thermodynamic layer with ocean heat transport prescribed using a heat-flux convergence field that varies with position and season but has no inter-annual variability.” The body of the paper closes with mention that future experiments will use “a model with a fully dynamic ocean”. With this kind of coupled ocean, it seems silly to be highlighting the prediction range of the current grand ensemble as if it had more than dubious predictive value.

Loehle has a pair of interesting articles which discuss the smoothing effect of combining time series, each with its own dating error. This is a reason not to be surprised when such multiproxy studies show relatively flat results. [Ecological Monitoring 171:433, Energy & Environment 15:1].

The 1995 IPCC conclusion you quote about anthropogenic influence should not be misconstrued, as it often is, as specifying anything about greenhouse gases.

[Response: We’ve mentioned the standards of Energy and Environment before. But your last point is bizarre. The IPCC 95, and the strengthened 2001 version, is indeed stating a probable anthro influence via GHG’s – William]

A few comments: In your eighth paragraph, the one that begins “In the spirit of this article”, you turned the corner from strictly discussing the science to discussing policy recommendations.

I really want to avoid discussing the policy debate, beyond injecting my own fundamental faith in human nature. We will not soil our own nest. The time may not have yet arrived when the “on the ground” reality of human influence on climate has sufficiently alarmed the general population. I think that is beyond question, given the very little that has actually been done “on the ground” to begin to reduce CO2.

However in the 50’s we banned atmospheric testing of thermonuclear weapons. In the 80’s we began to abandon the widespread use of CFC’s. If anyone can suggest to me why I should not use these two examples as a model for how humanity will (and should) respond to our release of chemicals into the atmosphere, I’ll be happy to listen. But comparing U238 and almost purely artificial CFC’s to a relatively much more benign and “natural” chemical (CO2), I think the human reaction to our fouling our nest with Carbon Dioxide should certainly be no more extreme than our reaction to these other two pollutants — we got the job done when the science succeeded in providing a political mandate.

So I wish all the advocates for immediate action the very best. Of course you are correct that reducing/controlling CO2 emissions is the wisest course of action to take immediately.
But the scientific community can and is contributing in many ways — providing alternative energy options, improving efficiency, etc. I have great hopes of seeing fusion become a viable energy source within my life time. (That would trump this discussion, I suspect.) When the public becomes sufficiently concerned, they will flock to these alternatives, and they will demand action of their politicians. Again, I have faith in human common sense.

Now, back to the science of hockey sticks. Frankly, this figure you presented might better be described as a “putter” (golf). Here’s my complaint: Michael Mann and colleagues have been exemplary in carefully detailing the 2 standard error uncertainty envelopes to their findings. Taking the ruler-straight handle of a hockey stick, and keeping it within this envelope, one can produce a wide range of linear trends that remain entirely within that envelope at all times — all the way from 200AD until 1980 when the envelopes end due to lack of further sufficient data to produce the “>40 year” smoothing of the proxy data.

The lines that can be drawn range from a temperature decrease from -0.05 in AD 200) to -0.10 (in 1980) along the upper edge of the envelope to a temperature increase from -0.50 in 200AD to -0.35 in 1980.

The pitfalls of producing statistical judgements are endless, however I just want to note that it is only right at the very, very end (within just a few years of 1980) that the instrumental record or any of the proxies, first emerge above any of the possible projected lines that can be drawn through the uncertainty envelope.

I do seriously question the motives of the authors in including, in bright red, a “smoothed” instrumental record which is not smoothed using the same “>40 year” smoothing which was applied to the proxies. In fairness for comparison purposes, and because a reasonable estimate of climate cannot be made without applying such a broad smoothing to yearly data, I do not feel it is appropriate for the figure to include a shorter term, more noisy, representation for the observations than are used to display the proxies. So I dismiss the tail of the red curve. I do this knowing full well that the numbers are real. But I dismiss them because they cannot tell us anything about the true long-term climate trends. And having dismissed most of the “blade” of the hockey stick, I am left with, as I said, more of a “putter”.

To summarize my perspective. I see the GHG climate signal “emerging” — but only just so. I understand the spirit of the detection and attribution work, which, in theory allows us to project the current trends based on their demonstrated cause using model results. But this is all theory — and I fear that some of the detection and attribution work as well as the modeling work, have suffered from not being double blind — if you know what you’re looking for, you’re much more likely to find it.

So from my perspective, the “on the ground” trend is the kind of solid evidence that the average person will react to. Inertia makes the public perception slow to turn. But if the trend continues as IPCC projects, and as nearly all reasonable scientists, including myself, expect, then the changes *will* get their attention. But I don’t subscribe to the use of scare tactics, even by those who are honestly scared. I reject the hype in a recent
article in the Independent about a “point of no return” for CO2 emissions. When CO2 emissions are reduced, climate will respond. Meanwhile the intriguing affairs of human politics will surely produce more catastrophic outcomes from other causes, unrelated to climate.

Regarding comment #4, I’ve asked the following question under other topics but have not gotten a direct response. Peter asserts his faith in humanity’s instinct not to soil it’s own nest. I would generally subscribe to that faith myself. The troubling aspect of climate change to me, as a layperson, is that, as others have analogized, it is like the frog placed in a pot of water that is brought slowly to boil. The frog doesn’t know that it should jump out. My rudimentary understanding of the accumulation of greenhouse gases is that they persist for a relatively long time. The problem with greenhouse gas emissions is that the problem is different in character compared to CFCs, etc.; there is a relatively short time between action and result. Therefore switching over to fusion or other non-fossil fuel sources decades from now will put us in the position of significantly higher atmospheric concentrations of GHGs that will take a long time to dissipate through natural processes (unless someone comes up with an effective sequestration method I suppose). And given that people have a very poor understanding of the stocks and flows nature of GHG’s (see John Sterman’s paper, “Cloudy Skies: Assessing Public Understanding of Global Warming”, http://web.mit.edu/jsterman/www/), I’m not as comfortable as Peter that there will be timely breakthroughs in technological fixes, public understanding, and policy.

So my question again is, how long can we afford to wait to reduce GHG emissions? I’m not asking this rhetorically. Can those of you who model climate shed any light, for us laypeople, on what happens if we wait 10, 20, 50, or 100 years to reduce emissions? Putting the effects of higher atmospheric concentrations aside, if we double, triple, quadruple CO2 concentrations, how long does it take to reduce those emissions? I’m assuming that if technological fixes come along, that society will still have to deal with decades or centuries of climate change impacts before we return to “normal” levels of GHGs in the atmosphere. Thanks.

Wigley and Raper (“Interpretation of High Projections for Global-Mean Warming,” Science, Vol. 293, 20 July 2001) assume a log-normal distribution for the IPCC TAR projections, and come up with the following temperature rise probability distribution for the year 2100, relative to 1990 (see “log + all”):

Michael Crichton’s 0.8 degrees Celsius prediction, as I understand it, is for the 21st century. So if we ratio that for the 110 years from 1990 to 2100, the value would be = 0.8 * 110/100 = 0.88 degrees Celsius. Let’s take that as Crichton’s “50% probability” value, and use the exact same ranges described by Wigley and Raper for the IPCC. For example, Crichton’s “75%” value would be 0.88 degrees Celsius + 0.72 degrees Celsius (which is 3.78 minus 3.06)…or 1.6 degrees Celsius. Completing the calculations for Crichton’s predictions:

I will bet Gavin Schmidt or any other author on this website $200 on LongBets.org that Michael Crichton’s projections for temperature increases are more accurate than the IPCC, assuming that the temperature being projected is average lower tropospheric temperature as measured by satellites. (With regard to the present debate about satellite measurements, I’m sure it will be resolved to the satisfaction of everyone by 2100.)

Just to make sure we all understand the terms of the bet, suppose the lower troposphere warms by 1.60 degrees Celsius from 1990 to 2100. According to Crichton, there’s a 25% chance that the lower troposphere will warm by that much. But according to the IPCC, there is only about a 4% chance that the lower troposphere will warm by that *little.* So if the lower troposphere warms by 1.60 degrees Celsius, Crichton and I would win the bet. Conversely, if the warming was 2.69 degrees Celsius (for example), the IPCC and Dr. Schmidt (or whomever else is willing to bet against Crichton) would win.

We’ll call this bet the “Julian Simon Memorial Bet.” ;-)

I’m waiting, gentlemen. And I’m not kidding.

Mark Bahner (environmental engineer)

[Response: There are at least three things wrong with this idea. First and foremost, neither you nor I will be around to collect, so this appears a little pointless. Secondly, Crichton did not assume the same uncertainty as IPCC, and in fact made his estimate to 7 or so decimal places. That you would win if temperature changes were 1.6 (twice his estimate, but within the IPCC range) seems a little perverse. Thirdly, we are not in the fortune telling business. The reason why different projections are used is precisely because we have no clue how technology or economic growth are going to change things 100 years down the line. However, given a particular increase in greenhouse gases, we can say reasonably precisely how much it will warm. The current energy imbalance at the surface (as demonstrated by the increasing heat content of the oceans) implies there is at least a further 0.5 deg C surface warming in the ‘pipeline’. As I said, keep your money in your pocket. – gavin]

[Response: I’m a bit more keen on this idea, though it remains to be worked out. Its been discussed on sci.env recently. But its a bit far out of RC’s remit. Go to sci.env, or my blog – William]

[Response:There’s another, more serious problem with this bet. All these scenarios are non-mitigation scenarios, i.e., they tell us what would happen if we did nothing to slow down or stop climate change. We’re unlikely to ever see the outcome of this hypothetical experiment. I think it’s very unlikely that humanity will sit and do nothing about the problem until 2100. I think it is quite possible, perhaps even likely, that warming by 2100 will be 1.6 degrees Celsius – but not because Crichton was right but because people recognised he was wrong and implemented measures to halt the warming. – Stefan]

You may well have addressed this at another place and another time, but at least in this sequence I’m left grasping.

You seem to argue explicitly that no evidence about what has happened in the past millenium is significant with regard to the conclusion that human caused global warming is upon us, because we can come to that conclusion based upon analysis of what has happened over the past century.

This arguement would seen to hold as well for the whole history of the earth.

That seems to raise an obvious queston: how is it that a relative eyeblink of time can be so definitive with regard to a warming trend that as far as I know has been significantly exceeded in the past?

[Response: The answer is very simple: if you want to study the causes of 20th Century warming, you need to look at 20th Century data. And not just the climate response, but more importantly, the forcings. Otherwise you cannot determine the cause. (E.g., to check whether solar variability may have caused 20th Century warming, you need to look at 20th Century solar data. Information on temperatures in the 15th Century doesn’t help much to answer that specific question, except for giving you some rather general information on possible climate system behaviour.) If you want to study the causes of a warming or cooling in period X of Earth history, you study the data from period X. – Stefan]

[Let me clarify this a bit. Stefan is not saying “we don’t learn anything from paleoclimate records”. To the contrary, we have learned a lot about “how the system works”. A particularly good example is that of abrupt climate change, as recorded in Greenland ice cores, which few scientists thought was likely, or even possible, prior to those discoveries.–eric]

Regarding #4
‘If anyone can suggest to me why I should not use these two examples as a model for how humanity will (and should) respond to our release of chemicals into the atmosphere, I’ll be happy to listen.’

Cause and effect in the two examples given were readily apparent, and the link between the two short and easy to grasp in the public mind; in my daily experience, this is far from true in the case of ‘global warming’.
Further, the two examples required no personal sacrifice or change of lifestyle on the part of the general population (concerned or otherwise), which seems to be unavoidable to curb emissions of GHG’s.; unless a way can be found to deliver ‘the goods’ to the world population, while at the same time reducing emissions drastically (can’t see that somehow!). ‘My little bit of pollution is insignificant’ – even countries reason that way. ‘I’m not going to/going to…whatever…and miss out until so-and-so next door does.’ I’ve seen no letup in rampant consumerism and selfishness in the last 50 years.
Finally, the lag, as I understand it, between cause and effect with respect to ‘global warming’ will probably mean that by the time mankind in general, and leaders in particular, are concerned enough to do anything meaningful, it will probably be too late.
Though I admire your faith in human nature, I fear that it is, sadly, futile.

An excellent site, just what was needed. I think information regarding emerging climate trends and their correlation to climate models would be useful (though, understandably, at this time tentative).

Re: The main reason for concern about anthropogenic climate change is not that we can already see it (although we can). The main reason is twofold….

Of the emitted CO2 from burning of fossil fuels, about 50% is absorbed by the ocean and terrestrial carbon sinks at present. See Sarmiento and Gruber. Although the “residence” time of a CO2 molecule in the atmosphere is about 5 years due to the photosynthesis/respiration flux and the atmosphere/ocean flux, the actual time that the excess CO2 remains in the atmosphere is long; it is thought to be about 100 years. This is due primarily to the fact that CO2 is non-reactive in the atmosphere and it takes quite a while for natural processes (mostly in the ocean) to scrub it out. Also, it thought that the “easy” carbon sinks are becoming saturated and will be less and less effective over time. This means that a greater percentage of future emissions will stay in the atmosphere.

Now, add to this some bad news about increased use of Coal as reported in New coal plants bury ‘Kyoto’. (Contains a quote from Gavin). Coal is “dirtier” than oil or natural gas with respect to CO2 emissions in a ratio of about 3 (coal): 2 (oil): 1 (gas). The main culprits (usual suspects?) are the U.S., China and India.

Now suppose the likely climate sensitivity for doubled CO2 is at the IPCC low end (1.5°C – which it isn’t, it is probably at least 2.0°C). Stefan says:

The IPCC gives the uncertainty range as 1.5-4.5 ºC. Only if this is wrong, and the true value is lower, can we escape the fact that unabated emissions of greenhouse gases will lead to the warming projected by the IPCC.

Well, given humankind’s renewed eagerness to burn fossil fuels and their long lifetime in the atmosphere, even a climate sensitivity below the low end estimate (which no one believes) or at the low end (which is highly unlikely) can still be overwhelmed by CO2 emissions going forward. How about a tripling (about 850 ppmv)? Anybody want to go for a quadrupling? I see that some people here like to gamble (#2, #6). Too bad (and how convenient) they won’t be around to see the results. But future generations will.

As regards a “fundamental faith in human nature. We will not soil our own nest.” (#4), I’ll believe that when I see it.

Thank you for another interesting and usefull post. It is important to remember now to get too bogged down in arguments over details when they don’t affect the big picture.

John Finn and Mark Bahner, thank you for an amusing end to a hard week of work. Wow 2100! Remember Julian Simon was an economist who knew something about what he was talking about, and Ehrlich didn’t represent a consensus among the scientific community or even demographers. You might want to be careful about relying on a work of admitted fiction when making predictions.

[Response: It is worth adding that Ehrlich has detailed a host of predictions that Simon would not bet on when Ehrlich offered. Ehrlich did this not because he thought “betting” was a useful way to produce good science, or good policy, but to show that the emporor (Simon) has no clothes. — eric]

Stefan writes: “Carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity. This is a measured fact not even disputed by staunch “climate skeptics.”

But he should have written more carefully. Indeed it appears that C02 levels are rising, but to interpolate “due to human activity” implies that “skeptics” believe in an anthropogenic cause. That’s plain wrong. So, now let’s consider the question of whether, on geological timescales, C02 trends anticipate or lag behind temperature trends.

[Response: It is unclear what you are saying is wrong here. That CO2 levels are caused by something other than human activity, or that the skeptics believe that CO2 levels are caused by something other than human activity? In any case, we’ve adddressed both the issues of CO2 causes and lags and leads in previous posts on RealClimate. See here and here . -eric ]

Re: 11. I have yet to hear any skeptic claiming that rising atmospheric CO2 is some sort of natural coincidence. If they are I don’t think they are a credible source. See this post here from Dec.22: http://www.realclimate.org/index.php?p=87

I’ll bet you guys at realclimate never anticipated this kind of response. The comment assumes that there is no evidence that fossil fuel emissions add CO2 to the atmosphere and that CO2 increases in the atmosphere may lag some natural variability that nobody can identify. Perhaps that variability is caused by Q-waves from the Galaxy Ignorancia 331A. An uncertainty in some ice core results is somehow “translated” into a statement about current CO2 emissions data in the 20th and 21st centuries (which can be measured precisely). However, Q-waves are not taken into account….

Now if you respond, that seems ridiculous: the true premise underlying the climate warming problem needs no defense. On the other hand, if you don’t respond, you leave the statement out there hanging in the wind for (perhaps) inhabitants of the galaxy alluded to above to pick up on.

If anyone can suggest to me why I should not use these two examples as a model for how humanity will (and should) respond to our release of chemicals into the atmosphere, I’ll be happy to listen.

I have to respond to this one, too. The reason I’m concerned that we won’t respond to this global threat in a rational way is that huge amounts of intellectual and writing talent are being exerted to make sure we don’t. This site is an important addition to the cause of pushing out accurate information. I don’t think the previous campaigns you mentioned, to stop nuclear testing or limit CFC use, had huge armies of paid writers and skeptics backed by organizations with political and economic interests in confusing and skewing the debate.

At some point, it had to be pretty obvious to a significant number of the stone head people that the trees were going to be decimated. But at a certain point (tipping point) it also probably became clear that the pooch was already screwed. And so, why not build a few more heads. After all, like our SUVs and trophy homes, they were a big status symbol.

Given a lot of the commentary on this site, I feel we may have already screwed our own particular pooch. Even if you are a believer, you reach the point where it all seems hopeless. So why not get on the band wagon to hell and enjoy the ride?

Especially scary to me was the comment that excess CO2 remains in the atmosphere for 100 years. What does this mean even assuming that we cut our emissions by, say, 50%.

Rearding the above comment in #9, I am still a bit confused with respect to the difference between residence time (5years) and “remain” time of 100 years. I would appreciate some clarification of that issue.

[Response: Residence time in the atmosphere refers to the average time before a molecule is reabsorbed into the ocean mixed layer. However, the ocean mixed layer equilibrates quickly to the atmospheric concentration, but has a very slow equilibration time with the deep ocean. So any anomaly in the atmosphere+ocean mixed layer takes a long time to disappear. This long time (>100 yrs) is essentially is the residence time for the atmopshere/ocean mixed layer combination of reservoirs. – gavin]

There is, of course, a wide spectrum of “greeness” among the governments of the world. And there is an equally wide spectrum of suppression and/or neglect of the populations they serve.

Governments do shift policy, sometimes rapidly. At other times change is agonizingly slow.

What is the most reliable way to convince each of the disparate governments to mandate reductions in CO2 emissions? Governments act in their own best interests first and foremost — they have as much instinct for self-preservation as individual organisms do. And like living organisms, the strongest motivators for change are usually greed, fear, and pain. Convince a government that CO2 reduction will cause them (and in the case of democracies, their populations) less pain and suffering, will provide them with greater security and safety, and will improve their economic condition.

Frankly I don’t think the pure science of the global warming issue has clearly identified enough of a global threat of “pain and suffering”, nor of “security and safety”. And the economic cost-benefit ratio argument has always been a difficult one in the absence of cheap, clean, widely available alternative energy sources.

Pain and suffering: Creeping sea level rise gets juxtaposed with tsunamis and with engineering marvels such as the Dutch Polders. Encroaching drought in the sub-tropics is juxtaposed with improved growing season length, CO2 fertilization, and potentially more precipitation for more polar nations. Etc., etc.

Security and safety: The current consensus among scientists is that sudden, catastrophic climate change is very unlikely. The threat of a generally more volatile and uncertain (insecure) climate is perhaps the strongest argument science has been able to make. And many governments have been convinced on the strength of these findings. Yet the science in this area still has much work to do. Extreme events are notoriously the most difficult phenomena to correctly represent in climate models. Progress is continuing apace; and when (I choose to use that word rather than “if”) the science becomes more robust, and when (or if) the corresponding climate trends toward volatility of weather emerge clearly from the background noise of “natural” daily weather, then more and more governments will find motivation to act.

Hopefully breaks in alternative energy technology will accompany the progress in climate science. That, coupled with the increased urgency which will (or might) arise if climate becomes more obviously destructive, should, in my view, eventually open the eyes of enough of the power brokers of the world so that meaningful policy actions will begin to be implemented.

With respect to residence time in the atmosphere and mixed layer of the ocean, most carbon cycle models I’ve seen also include the biosphere, and the residence time among these three is often given as much longer than 100 years but shorter than a millenium. My question is, what is the average time for the excess atmospheric CO2 produced by burning fossil fuels to mix into the deep ocean reservoir.

[Response: The mixing into the deep ocean is not a simple process; it is enhanced by the ‘rain’ of organic matter through the ocean column, but decreased by the ‘rain’ of carbonate shells. It is enhanced too by the formation of deep water in the polar regions, but slowed by the warming of the surface ocean. The complexity of these processes mean that the decay of the CO2 impulse into the atmosphere/ocean mixed layer cannot be characterised by a single exponential time scale. The best estimates are for the average are therefore a little uncertain and range between 50 and 200 years. – gavin]

If I understood your answer, it is not the answer to the question I asked. Perhaps I was not clear. I accept that the mixing into the mixed layer is 50-200 years. However, the mixed layer being accessible to turbulent wave mixing with the atmosphere, is essentially in contact with the atmosphere, and over fairly short times equilibrates with it and other parts of the biosphere. Moreover, the amount of carbon in the three reservoirs, the atmosphere, mixed layer and the surface biosphere are approximately equal. Given this situation, a pulse of CO2 injected into the atmosphere will fairly quickly relax to ~ 1/2 – 1/3 of the injected value. This is what we have seen.

My question was how long it would take for the excess injected into the upper three reservoirs to relax into the much larger deep ocean reservoir. Only when this happens will todays observed increase in CO2 mixing ratio decrease. It was my impression that this was the 500-1000 year process (admittedly it is not a single process, but a complicated melange).

Just to be clear, I am not talking about incorporation into rocks, which will take much longer, and be the final dilution.

[Response: Maybe I wasn’t clear; the mixing time for the atmosphere/ocean mixed-layer to come into equilibrium is short (around 5 years), the longer time-scales of 50-200 years are for the mixing into the deep ocean and are thus the appropriate time scale for your question. There is a long ‘tail’ of the response (>1000 years) due to the slow mixing of deep ocean waters which makes it dificult to give just one number. As you say the timescale for full sequestration is more like ~50,000 years. – gavin]

This is a well-written piece that covers the issue very well. However, the wording of the sentence: “the point of conducting science is to give an early warning, rather than just wait until the facts are obvious to everyone” is the weakest part of the entire argument. Not surprisingly, today I read an editorial by Mark Milke in the ‘Calgary Herald’, exploiting this weakness for maximum rhetorical effect. The editorialist removes the sentence from the context of the preceding text, and then argues: ‘Readers should examine the above point by Rahmstorf because it contains shaky reasoning. If it is indeed the case, as McKitrick and McIntyre claim (that the science behind Kyoto is not solid), then to argue that the ‘only thing’ that proves is that ‘our data are not yet good enough’ assumes the very conclusion one is trying to reach.’ He then goes on to say that ‘McKitrick and McIntyre’s article ‘undermines claims about what caused late 20th century warming’ (essentially ignoring most of the points made above by Rahmstorf).

I wrote a letter to the editor to try to expose some of these tactics and to more clearly convey the original message of this post. It will be interesting to see if they publish it. To give the editors some credit, they did state at the bottom of the op-ed piece that “an associate of Michael Mann has promised to send a response to the McKitrick and McIntyre allegations in the coming days. Ther position is outlined at: http://www.realclimate.org and http://www.climate2003.com“

I’m sorry, but your response still doesn’t have me quite with the program.

Indeed yes, one must look to 20th century events (approximately) to understand 20th century outcomes. But given what I understand to be true, that greater warming has occured than in the distant past than is currently occurring, how can we be so sure we are examining all the right 20th century events, since these earlier warmings were clearly caused by events other than human driven carbon dioxide emissions?

Further to 12 and eric’s response: I’m sorry if I wasn’t clear enough: in the second sentence by Stefan that I quoted, “this” can only sensibly refer back to the whole of the first sentence, giving the reading That carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity is a measured fact not even disputed by staunch “climate skeptics.”‘.

Further to 14 from dave, I’m happy for him that he has the power to measure ignorance from such a distance.

Directed to Jo (#23): What the “skeptics” tend to dispute is that the warming seen in the 20th century is mostly due to anthropogenic causes (and the rise in CO2 in particular). They do not tend to dispute that the rise in CO2 is due to anthropogenic causes.

The evidence that the CO2 rise is due to humans is incontrovertable. For one thing, the timing with the industrial revolution is hard to dismiss as a coincidence, especially since it is known that CO2 levels haven’t been as high as they are now for at least ~1 million years (over which we have very good data from ice cores) and likely for the last 20 million years.

Secondly, the amount of the concentration rise is in line with the amount we know we are putting into the atmosphere through the burning of fossil fuels. (Actually, the rise in CO2 concentrations is generally running at a rate of about 1/2 of what we know we are adding to the atmosphere, showing that the biosphere and hydrosphere is able to take up some but not all of our additions.)

Thirdly, I believe that by looking at the different isotopes of carbon and/or oxygen in the CO2, the additional CO2 can be attributed directly to the burning of fossil fuels.

Of course, what is true is that as we warm the atmosphere, the not-well-understood processes by which CO2 and methane seem to get released as the earth warms may come into play again and create a positive feedback. If this occurs, we will start to see an acceleration in the increase in concentration of CO2 and it could even start to rise at a rate greater than the amount we ourselves are directly adding to the atmosphere.

Suppose that scientists are completely wrong and that no future climate change will result from increased CO2 in the atmosphere (of course we must ignore basic laws of atmospheric physics for this, but let’s for the sake of argument assume that this were true). Then, surely, there would be no reason for concern over greenhouse gas emissions?

This might be true when only considering climate change impacts. But CO2 is accumulating in the OCEAN also, causing changes to ocean pH and biogeochemical cycles, and thereby posing risks to marine ecosystems, coral reefs, and fisheries resources. In recent years, many papers in ‘Nature’ have been published on this topic (e.g.: Kleypas (1999), Riebesell et al. (2000), Caldeira and Wickett (2004)) (also see this paper: http://www.stabilisation2005.com/55_Jerry_Blackford.pdf). Decreased calcification of corals due to higher aqueous CO2 is already occurring-and coral reefs are important for fisheries, tourism and recreation, coastal protection, and pharmaceuticals.

It seems to me that this alone is enough reason for concern that greenhouse gas emissions pose risks to society and nature. These risks are completely independent of any of the risks related to climate change. One could even imagine a hypothetical situation where the scientific consensus stated that human-induced climate change is of no concern, but that the long-term risks of CO2 accumulation in the ocean are reason enough for multi-lateral international agreements to reduce CO2 emissions.

Also, it thought that the “easy” carbon sinks are becoming saturated and will be less and less effective over time. This means that a greater percentage of future emissions will stay in the atmosphere.

There is an interesting discussion of this by Chris Jones, et. al. of the Hadley Centre Impact of Climate-Carbon Cycle Feedbacks on Emission Scenarios…. The paper is part of the current “Avoiding Dangerous Climate Change” conference happening now in England. The discussion talks explicitly about how diminishing terrestrial and ocean carbon sinks over time require reduced CO2 emissions from fossil fuels/land use to achieve stabilization goals at various levels (e.g. 550 ppmv of CO2 in the atmosphere).

The one thing you true believers in this debate refuse to acknowledge is that, if we (the rest of the human race) are to take action, what is your prescription for this action? If we accept that human CO2 on the order of parts per billion causes warming, it begs the question of what we should do to cause a cooling.

If I crash my car, the fix is not to back up – the damage is already done – nor is it to go by a different route. The means to undo the damage is completely different than the process that caused the damage in the first place. Since meaningful diminution of global CO2 emission is either technologically impossible or is beyond the amount of sacrifice that the human race is willing to undertake, why does the IPCC absolutely refuse to even consider looking into means to counteract the effect of CO2? Or even to discuss methods of reducing CO2 other than a reduction in emissions?

The unanimous rejection of Kyoto by the U.S. Senate should show you guys that, until you attempt to posit a solution to warming that will not impoverish this country, it will do nothing to ameliorate your concerns. Science is all about possibilities and probabilities. Until the scientific establishment gives policy makers a range of choices to choose from as a response to your scenario, rather than your one solution (CO2 emission reduction) you are practicing politics, not science. And not very well at that.

In reply to comment #27: I am actually rather confused by your post. First, you ask for a prescription and then you say that the scientific establishment should not give prescriptions but should instead talk about the range of choices available. If you actually read the parts of the IPCC report discussing adaptation and mitigation, rather than just saying what you have been led to believe the IPCC says, you would find that they do exactly what you want them to do…and what I agree they should do.

You also make the usual assumption of naysayers of naively believing any “sky-is-falling” economics assumptions you hear from special interests in regards to the cost of Kyoto and similar measures. Did you know that British Petroleum has implemented Kyoto-sized cuts in its own emissions 8 years ahead of schedule and claims to be saving hundreds of millions of dollars (per year, I believe)?

At any rate, in my personal view, we should not prescribe exactly what needs to be done but should instead implement flexible schemes like Kyoto or the McCain-Lieberman Climate Stewardship Act that allow trading of emissions credits, credits for carbon sequestration (provided it can truly be shown to work) and so on. What we should not do, however, is to continue to pretend that there are no costs associated with our greenhouse gas emissions. The market is indeed the best way to come up with solutions but in order for the market to find solutions, it needs to know there is a problem which you do by requiring the cost of the emissions to be borne directly by the emitters.

[And, a technical point, the CO2 levels are hundreds of parts per million not parts per billion.]

Michael Gersh said cutting greenhouse gases via Kyoto will “impoverish this country” .. I would argue instead the transition to alternative ways of living promises to spark a whole new technological revolution and enrich people the world over. The only people worried about “impoverishment” are the conservative elite who stand to loose when the establishment gets shaken up. While the rest of the world moves forward with Kyoto, while Germany becomes the worlds largest Solar producer, and so on, the USA is being left behind, the gap is widening. That is the real impoverishment. The more socialized countries are able to move quicker on this, they are not so controlled by the special interests who are so worried about short term profit unable to take the long view. In the USA it will take the consumer to change first, by buying green, it’s allready happening.

RE:27. Firstly, I don’t think it’s true that nobody is examining solutions other than emission reductions. Secondly, US politics is already showing small signs of change-for example, the McCain-Lieberman Climate Stewardship Act almost passed in the Senate, and California and some eastern states are moving forward with state policies for energy efficiency and cap-and-trade schemes. Third, the contention that reducing emissions will cause ‘impoverishment’ is pure speculation and fear-mongering. Was the US ‘impoverished’ in the year 1990, when emissions were much lower? Would more efficient public transit and increased telecommuting lead to ‘impoverishment’, or rather saved money, improved health, and more time to spend with family and friends? Would the development of wind, solar, and tidal energy industries create less jobs than coal mining? If your car’s engine were 30% more efficient, would you complain about the extra pocket money you had from fuel savings? If more freight were moved by rail than by truck, would it make any difference to you as a consumer? The list of such questions is very long indeed.

Stefan Rahmstorf writes, “1) Carbon dioxide and other greenhouse gases are increasing rapidly in the atmosphere due to human activity. This is a measured fact not even disputed by staunch ‘climate skeptics’.”

Well, I don’t know who these “climate skeptics” are, but *I* dispute it. Because unless one has a very bizarre definition of “increasing rapidly,” your assertion is flat out wrong.

CO2 is increasing by approximately 0.5 percent per year. Is that “increasing rapidly?” I don’t think so!

Methane concentrations appear to have plateaued…or are plateauing. In fact, there’s probably close to a 50/50 chance that the methane atmospheric concentration a decade from now will be LESS than at present.

[Response: Possibly he means rapidly as in ‘more rapid than at any time over the ice core records’? At the end of the glacial periods, CO2 increases about 80 ppm over ~ 10,000 years. That is 0.008 ppm/year. 1.9 ppm/year (over 200 times faster) would appear to me to be pretty rapid. The same is true for CH4 and CFCs over the 20th Century, but as you point out correctly, the growth rate of CFCs is expected to slow (though as a result of the Montreal Protocols). The growth rate of methane has indeed slowed, and this is very good news; unfortunately, we don’t quite know why. It remains however over twice as high as in pre-industrial times. The key issue for climate is whether the net forcing from these gases is likely to continue to grow rapidly and, since this is dominated by the CO2 changes, the likelihood is that it will. – gavin]

[Additional Response: If I may clarify this further: YES 0.5 % per year is rapid. But Stefan’s point was actually not about the rapidity, but about the fact that the increase is due to CO2 production by humans. Both of these are well established from measurements. They are not matters of opinion of inference. See our earlier post on this– eric]

Having read the comments here and in Jones and Mann (2004) ‘Climate over past millennia’, I have been reflecting on some of the comments about the hockey stick wrt the Medieval Warm Period (MWP) and the Little Ice Age (LIA). I accept that these terms are often used too loosely, that they encompass regional rather than truly global events, and that they carry a Eurocentric bias. At the same time they are real at least in as much as they are measurable by wide scale (beyond the local), sustained (more than decadal) physical, economic and social yardsticks (real people suffered due to the LIA). I also reject Jones and Mann’s implication (p31) that only hemispheric- and global-scale series are ‘true’. They show no more than they conceal (large regional variations and variations obscured by 40 yr smoothing).

The quest for the ‘single figure’ that shows anthropogeneic warming ignores the fact that (with the possible major exception of sea level) the direct impact of climate change will vary between regions and climatic zones.

My question is this. Have any serious attempts being made to construct an index of global climate variation (as opposed to mean) for the last 1000 years (regional and interdecadal) ? My basic stats knowledge tells me to be wary of a mean without a standard deviation or some other measure of variance. True, it could be hard to do this for methodological reasons, but I have a suspicion (and only a suspicion) that it would show more substance to the MWP and LIA events than some credit them (although I’d be happy to be proved wrong). It might also help headline writers focus on something more meaningful than crude global mean figures when interpreting global warming findings.

Finally, allow me to thank you for this excellent site.

[Response: Your point is extremely well taken. The patterns of regional climate change are likely to me more useful for understanding mechanisms and intrinsic climate variability than simply the mean hemispheric temperature, which clearly averages over a lot of structure. Some reconstruction methodologies do already give regional patterns though; MBH98, Luterbacher et al (2003), Rutherford et al (2005) etc. and these are being used (for instance in Schmidt et al (2004)) to better match the model’s responses to different forcings (solar and volcanic particularly). This is probably more likely to be scientifically significant than the ‘single’ mean number produced by any of them, despite the fact that this seems to get all the attention. -gavin]

WRT #34, thanks for your answer Gavin, the Schmidt reference was interesting. My question was more aimed at seeking variation figures in the instrumental or proxy record rather than via modelling. Perhaps this is expecting too much!

[Response: I pointed it out mainly so that you’d notice the comparisons (i.e. figures 3,4 and 5) which all have spatial patterns derived from the proxy reconstructions. There are some more interactive resources (that might be more to your liking) associated with the Mann et al (2000) Earth Interactions paper. You can pick different years (back to 1730) and see what the reconstructions look like. – gavin]